A gas turbine engine generator system is currently a preferred form of generator system in various fields, and, in particular, is considered to be suitable for use in a cogeneration system because the waste heat which the gas turbine engine produces can be readily recovered for use as a heat source for space heating, utility hot water and other useful purposes.
When the gas turbine engine is operated so as to maintain the internal temperature of the engine at a limit operating temperature, a high efficiency and a minimum emission can be both achieved. Therefore, when the load of the engine is increased, it is desirable that the rotational speed of the engine is increased while maintaining the internal temperature of the engine at the limit operating temperature. In other words, when the demand for electric power increases, the supply of fuel to the engine is required to be increased in such a manner that the rotational speed of the engine may be increased to a target rotational speed that matches the increase in the demand for electric power or the load while controlling the internal temperature of the engine so as to maintain the efficiency of the engine and the emission level at an acceptable level.
A gas turbine engine cannot be accelerated as rapidly as desired because the inertia mass of the rotating members resists acceleration. Also because the engine is operated with the internal temperature near the limit operating temperature all times, the margin for increasing the supply of fuel is small, and this prevents a rapid acceleration of the engine. Therefore, when a demand for electric power suddenly increases, a certain time lag is inevitable before the rotational speed of the engine reaches the target rotational speed that matches the increased demand.
When there is a need to match the output of a gas turbine engine generator to a demand without any such time lag, it has been practiced to use a battery to fill the shortage of the power output while the gas turbine engine accelerates. According to this method, as illustrated in FIG. 5, the difference between the demand and the output is simply filled by the supply of electric power from the battery.
However, according to this method, because the gas turbine engine operates under a limit condition the whole time, it takes a relatively long time for the rotational speed of the gas turbine engine to reach the target value. Therefore, the battery is required to supply electric power for a correspondingly long time so that the total amount of electric power (as indicated by the hatched area in the graph of FIG. 5) which the battery has to produce is significant, and a correspondingly large battery is required.
It is also known from WO99/32769A1 to temporarily stop the generation of electricity by the gas turbine engine while the engine is being accelerated and supply the entire demand solely from the battery during this process as illustrated in FIG. 6. According to this method, the gas turbine engine can accelerate relatively rapidly so that the time period during which the battery has to supply electric power is significantly reduced and the total amount of the electric power that the battery has to supply is somewhat reduced. However, the battery still has to be large enough to be able to produce a large current and meet the entire demand. Also, repeated discharge of large currents is known to reduce the service life of the battery, and this increases the running cost of the system. Furthermore, the absence of load during the time of acceleration lowers the internal temperature of the engine, and this temporarily causes a drop in the engine efficiency and an increase in the emission.
The battery used for this purpose typically consists of a lead battery which is relatively heavy and bulky. Therefore, the reduction in the requirement of the battery is highly essential for an overall compact design of a gas turbine engine generator system. As a lead battery is required to be replaced every now and then, the need for a large battery means an increased running cost.